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Anal. Chem. 1985, 57, 2280-2282
Direct Liquid Introduction Interface for Capillary Column Liquid ChromatographyIMass Spectrometry Takao Tsuda* Laboratory of Analytical Chemistry, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466, Japan
Gerhard Keller and Hans-Jurgen Stan* Institut fur Lebensmittelchemie, Technische Uniuersitat Berlin, Muller Breslau Strasse 10, 1000 Berlin 12, Federal Republic of Germany
The total effluent from a packed mlcrocaplllary column, operating at a flow rate of 0.2-0.4 pL/mln, has been Introduced Into the Ion source of a quadurapole mass spectrometer. As the amount of effluent Is comparatlvely small, the pressure In the Ion chamber Is observed to be around lo-’ torr and electron Impact Ionization becomes posslble. The Interface was constructed from commerclal flttlngs. The transfer llne conslsted of a fused silica Capillary (25 pm 1.d.) mantled by a stalnless steel tube (0.3 mm i.d.) whlch served as a protector and heat transferrer. Wlth a properly adjusted temperature of the Ion source block, dlrect llquid Introduction of the effluent can be achleved. LC/MS has been demonstrated by applying the herbicides prometon and proparlne as solute. The mass spectra monitored exhibit all features of electron Impact lonizatlon.
The coupling of gas chromatography with mass spectrometry (GC/MS) is one of the most powerful tools in many analytical fields. This holds true in particular for the direct connection between high-performance capillary GC with MS. In this case the effluent from the GC column is introduced directly into the ion source without any contact to materials other than deactivated fused silica or glass. On-line coupling of liquid chromatography and mass spectrometry (LC/MS) is carried out with four popular approaches: (1)moving belt, (2) atmospheric pressure ionization (API), (3) thermospray, and (4) direct liquid introduction (DLI). The moving belt interface was introduced by Scott et al. (1). It has been commercially available, and the advantage is that the solvent is removed before the sample reaches the ion source (2). API LC/MS was invented by Homing et al. (3). In this case, a corona discharge or 63Nifoil as @-sourcehas been used as a modified ion source. The thermospray technique was developed recently by Vestal and co-workers (4). It provides a special “soft” ionization, capable of producing intact molecular ions from large, nonvolatile molecules. A major disadvantage of these interfaces is the high price of the commercial devices available. Direct introduction of the effluent from a liquid chromatograph into the ion source was first described by McLafferty and co-workers (5). They introduced only a small fraction of about 1% of the liquid, and the solvent acted as the ionizing reagent for chemical ionization. A review on the optimization of the instrumental parameters was given by Arpino and Guiochon (6). LC/MS interfacing was the subject of several reviews (7-10). In recent years considerable efforts have been made to improve LC/MS interface. These works include modifying the moving belt system (11-13),thermospray interface (14-16), and nebulizing interface (17-19). And more recently, a 0003-2700/85/0357-2280$01.50/0
electrospray interface (20) and a monodisperse aerosol generation interface (21) have been proposed. The latter device combines DLI with the evaporation of the solvent. This permits the operation of the mass spectrometer in both electron impact (EI) and chemical ionization (CI) modes. Microbore column technology offers a very simple approach for LC/MS coupling comparable to capillary GC/MS. There, the low flow rate of eluent permits the direct introduction of the effluent into the ion source because the pumping capacity of the mass spectrometer can overcome the influx. Henion and Maylin (22) have shown that the total effluent from a microcolumn can be introduced into the ion source of a quadrupole mass spectrometer. The solvents act under the conditions as reactant gases. Schafer and Levens (23)reported on extensive testing of an interface, based on their concept. They introduced solvents at flow rates up to 15 pL/min and applied CI mode. Recently, Bruins and Drenth (24) constructed a simple interface, and operated their micro liquid chromatograph at a flow rate of 8 pL/min using the solvent as a reactant gas. The interface was consisted from flexible fused-silica capillary tubing and a heated block, which was in contact with the capillary. If we use a capillary column which has a smaller inner diameter compared to that of a microcolumn (W),the interface of DLI LC/MS using a capillary column would become simpler and the spectra obtained might be E1 mode. There are currently three different types of capillary columns: slurry packed capillary columns (26-28), packed microcapillary columns (29-31), and open-tubular capillary columns (32,33). Slurry packed capillary columns (0.1-0.3 mm id.) have been operated at the flow rate of about 1pL/min (26-28). The flow rates of a packed microcapillary column (e.g., 40 pm i.d.) (30, 31) and open-tubular microcapillary column (e.g., 10 pm i.d.) (32), a t a linear velocity of 1 cm/s, are around 0.7 and 0.05 pL/min, respectively. More recently, Stenhagen used a slurry-packed capillary column for LC/MS (34). In the present paper, we report on the coupling of packed microcapillary columns (PMC) with liquid flow rates between 0.1 and 1 pL/min to a Finnigan 4000 quadrupole mass spectrometer. Our interface is similar to that of Bruins and Drenth (24), but it is even simpler and smaller in size. Our preliminary results show that with this simple interface, a quadrupole mass spectrometer can be operated under E1 conditions.
EXPERIMENTAL SECTION A quadrupole mass spectrometer, Finnigan Model 4023 (Sunnyvale, CA) with an INCOS data system was connected to a capillary LC system, which consisted of a conventional pump, Gynkotek Model 600 (Munchen, FRG), a split injector, and a packed microcapillary column. This capillary LC system was nearly the same as those discussed in previous reports ( 3 0 , 3 5 ) . The split injector part consisted of a six-way valve with a 20-pL 0 1985 Amerlcan Chemical Society
ANALYTICAL CHEMISTRY, VOL. 57. NO. 12. OCTOBER 1985
Figurn 1. Schemaik diagram of the mlcrocapillary LClMS interface (not to scale): 1. Ion source; 2. heated Ion source block: 3. to quaaupole analyzer; 4, OC in; 5, SOlM pmbe in; 6, flange; 7, butt weki c o n m a ; 8, a088 (all c o n n e h female); 9, reducing unbn io ' I , # In.; 10, calibration ges in; 11. piranl gauge (pressure monRorlng); 12, epoxy glue. connectionof fused-silicaCapillary to stainless steel tube: 13, stainless steel tube. 0.3 mm id. and 0 6 mm o.d ; 14. fused-siika capwry (iramfer the),25 pm i.d. X 25 an;15, W walled PTFE tube. 0.1 mm i.d. X 10 mm; 16. HPLC packed mlcrocapillary.
loop, Rhecdyne No. 7125 (Contati, CA), a union tee (Swagelock), and a fme metering valve, Nupro Co.(Willoughby, OH), or narrow capillary tubing for flow resistance. The six-way valve and union tee were combined hy using short stainless steel tubing, ca. 3 cm in length, whose inner diameter was enlarged from 0.2 mm to 0.8 mm at the end part. The head part of PMC was inserted into the end part of the stainleas steel tubing in the split injector. The split ratio was adjusted between k500 and 1:loOO with a flow rate of 0.1 or 0.2 mL/min of eluent delivered by the pump. Thus a flow of 200 to 400 nL/min entered the ion source of the mass spectrometer via PMC. Of course this system can he improved by utilizing special micro-LC equipment. The effluent was transferred into a 25 pm i.d. fused-silica capillary tubing, S.G.E. (Melbourne, Australia), which was connected to the ion source via the modified calibration gas inlet. A schematic diagram of the LC/MS interface is shown in Figure 1. The only modification of the mass spectrometer was the exchange of the elbow-shaped connector for the calibration gas inlet with a butted, welded straight connector. This stainless steel welding was done in a workshop, hut all other fittings used (tube cross, tube adaptors, and reducing unions) are standard (e.g., Cajon, Swagelock) and allow flexibility for further modifications. A stainless steel tube (Figure 1, 13) protected the fused-silica transfer line (Figure 1, 14) which protruded 3 mm, unmantled. The tip of the transfer line was adjusted to about 20 mm from the ion chamber to bring about desolvation of the sample on the way to the ion source. Heat from the ion source block was transferred to the fused-silica capillary, via the stainless steel tuhe. The other end of the transfer line was connected to the capillary column simply by a s m a l l piece of thick-walled Teflon tube with an inner diameter of 0.1 mm onto both ends. Capillary LC was carried out with a PMC packed with silica gel, ca. 2 m X 50 pm i.d. (30,31). The eluent consisted of hexane (94), methanol (3), cyclohexanol (3), and water (0.005). The eluent was throughly degassed before use.
RESULTS AND DISCUSSIONS One of the main problems with DLI LC/MS interfacing, is considered to be the fluctuation of the spraying of liquid from the transfer capillary line into the ion source. A stable source pressure, however, is essential for reproducible mass spectra. Therefore, our first series of experiments were performed to check the ion source pressure as a function of temperature and flow rate. The temperature of the ion source block was stepwise changed from 40 to 240 "C. Two stable regions were observed, below 70 "C and between 140 and 170 OC. The flow rate of eluent was not critical in our experimental conditions. To avoid fast deterioration of the ion source, a temperature of 160 "C was chosen. The constancy of the effluent spraying was controlled by monitoring a characteristic ion of low intensity fmm the eluent mixture (e.g., the molecular ion of cyclohexanol, m / z 100). This mass chromatogram is a better indicator for pressure variations than recording the relative slow Pirani gauge readings. In Figure 2, typical RIC and MS chromatogram are shown. A in Figure 2 was by FUC,
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I,*.
I".
B
Flgur 2. RIC and mass chromatograms of
effluent: effluent, the mixture of hexane. m3ihanol. cyclohexanol, and water; solute, nitrobenzene; column, PMC. 47 pm i.d. X 220 cm; temperature of ion chamber, 160 'C. The chmmatopms of A. E, and C wwe RIC. mlz 99-102 (full scale 11 408 count) and mlz 99-102 (full scale 448 count). respectively. The mass chromatogam of C was obtained by substrading 10960 count from E.
and B and C were by ion current of m / z 99-102. C in Figure 2 (full scale was 488 count) shows just the upper part of B. The fluctuation of the count number of m / z 99-102 is only 5%. A similar rate of fluctuation of the count number of m/z 86, which corresponded to hexane, was obtained. So i t is suggested fmm the above results, that spraying by the present interface has been performed quite well. The temperature region, given stable ion source pressure, mbe ohtained in a reasonably short time for selected eluents. The small influx of liquid through the transfer line can be sprayed out without a heat exchanger as described by Bruins and Drenth (24). Their interface was constructed for a flow rate of 8 pL/min which is a t least 10 times more than that of the present experimental condition. In the thermospray interface for LC/MS devised by Vestal (14), sufficient heat is transferred to effect nearly complete vaporization of the liquid in a stainless steel capillary tubing (0.15 mm i.d.). In our device, the vaporization of the effluent would be restricted within only a small portion in the capillary, and then the vapor in the capillary would force the other portion of the effluent out with high speed, following spraying. We call this process %praying by partial vaporization (SPV)". The LC/MS system was applied to the analysis of two thermolabile triazines which have a similar structure and are in common use as herbicides. In Figure 3 the total ion current and the reconstructed mass fragmetcgrams for the separation of propazine and prometon are given. Each amount was ca. 10 ng. The m885 spectra of prometon and propazine are shown in Fimutes 4 and 5. We find that they are typical E1 spectra after Having compared them with E1 spectra from the NBS library. The respective patterns are quite similar. In the masa spectra, no M + H ions or other indications of the chemical ionization mode were observed. This means that, under our experimental conditions, solvents do not react as CI reagent gases. This interface is in accordance with the observed ion
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 12, OCTOBER 1985
(31). Therefore, the interface was not blocked by supports. The important features of the DLI LC/MS system described here are as follows: (1)It offers the possibility of free choice of the ionization modes on-line as commonly used with GC/MS. (2) The interface can be constructed easily with standard fittings. (3) The DLI interface can be permanently installed together with a GC/MS interface and a usual solid probe, as shown in Figure 1. The obstacle to application of the present method as an analytical tool seems to be the capillary LC system with a 50 ,um i.d. capillary column, which has a great potential but is yet in a developing stage. Registry No. Prometon, 1610-18-0;propazine, 139-40-2.
n
)j
LITERATURE CITED Figure 3. LC/MS recording of two herbicides.
225
PROMETON SAMPLE
168
L I B R A R Y DATA 168 183 112
14'
I
225
I
I
153
Flgure 4. Corresponding mass spectra of prometon from the LC/MS run in Figure 3 and from the NBS library obtained from a computer search.
(1) Scott, R. P. W.; Scott, C. 0.; Munroe, M.; Hess, J., Jr. J. Chromatogr. 1974, 99,395-405. (2) McFadden, W. H.; Schwartz, H. L.; Evans, S. J. Chromatogr. 1978, 122,386-396. (3) Horning, E. C.; Caroll, D. I.; Dzidic, I.; Haegeie, K. D.; Hornlng, M. G.; Stillweii, R. N. J. Chromatogr. Sci. 1974, 12, 725-726. (4) Blakeiy, C. R.; Vestal, M. L. Anal. Chem. 1983, 55, 750-754. (5) Baldwin, M. A.; McLafferty, F. W. Org. Mass Spectrom. 1973, 7, 1111-1112. (6) Arpino, P. J.; Guiochon, G. J. Chromatogr. 1982, 251, 153-164. (7) Kenndier, E.; Schmid, E. R. I n "Instrumentation for High Performance Liquid Chromatography"; Huber, J. F. K., Ed.; Elsevier: Amsterdam, 1978; pp 163-177. (8) Arpino, P. J.; Guiochon, G. Anal. Chem. 1979, 51, 682A-701A. (9) McFadden, W. H. J. Chromafogr. Sci. 1979, 17, 2-16. (10) McLafferty, F. W. I n Biochemical Applications of Mass Spectrometry"; Waiier, G. R.. Dermer, 0. C., Eds.; Wiiey: New York, 1980; 1st suppi. vol., pp 1159-1168. (1 1) Hayes, M. J.; Lankmayer, E. P.; Vouros, P.; Karger, L. B.; McGuire, J. M. Anal. Chem. 1983, 55, 1745-1752. (12) Hayes, M. J.; Schwartz, H. E.; Vourous, P.; Karger, B. L.; Thruston, A. D., Jr.; McGuire, J. M. Anal. Chem. 1984, 56, 1229-1236. (13) Hardin, E. D.; Fan, T. P.; Blakiev. C. R.: Vestal. M. L. Anal. Chem. 1984, 56, 2-7. (14) Vestal, M. L. Anal. Chem. 1984, 56, 2592-2594. (15) Covey, T.; Henion, J. Anal. Chem. 1983, 55, 2275-2280. (16) Voyksner, R. D.; Bursey, J. T.; Peilizzarl, E. D. Anal. Chem. 1984, 56, 1507-1514 - . (17) Apffei, J. A.; Brinkman, U. A. T.; Frei, R. W.; Evers, E. A. I.M. Anal. Chem. 1983, 55, 2280-2284. (18) Tsuge, S.:Hlrata, Y.; Takeuchi, T. Anal. Chem. 1979, 51, 166-169. (19) Yoshida, Y.; Yoshida, H.; Tsuge, S.; Takeuchi. T.; Mochizuki, K. HRC CC,J . High Resolut. Chromatogr Chromatogr. Commun . 1980, 3 , 16-20. (20) Whitehouse, C. M.; Dreyer, R. N.; Yamashita, M.; Fenn, J. B. Anal. Chem. 1985, 5 7 , 675-679. (21) Wiiioughby, R. C.; Browner, R. F. Anal. Chem. 1984, 56, 2626-2831. (22) Henion, J. D.; Mayiin, G. A. Biomed. Mass Spectrom. 1980, 7 , 115-12 1. (23) Schafer, K. H.; Levsen, K. J. Chromatogr. 1981, 206, 245-252. (24) Bruins, A. P.; Drenth, B. F. H. J. Chromatogr. 1983, 271, 71-82. (25) Ishii, D.; Asai, K.; Hibi, K.; Jonokuchi, T.; Nagaya, M J. Chromatogr. 1977, 144, 157-168. (26) Takeuchl, T.; Ishii, D. J. Chromatogr. 1981, 2 1 8 , 199-208. (27) Hirata, Y.; Jinno, K. HRC CC,J. High Resolut. Chromatogr. Chromatogr. Commun. 1983, 6 , 196-199. (28) Novotny, M.; Hirose, A.; Wisler, D. Anal. Chem. 1984, 5 6 , 1243-1 248. (29) Tsuda, T.; Novotny, M. Anal. Chem. 1978, 50, 271-275. (30) Tsuda, T.; Tanaka, I.; Nakagawa, G. J. Chromatogr. 1982, 239, 507-5 13. (31) Tsuda, T.; Tanaka, I.; Nakagawa, G. Anal. Chem. 1984, 56, 1249-1 252. (32) Tsuda, T.; Nakagawa, G. J. Chromatogr. 1983, 268, 369-374. (33) Ishii, D.; Takeuchi, T. J. Chromatogr. Sci. 1980, 16, 462-472. (34) Aiborn, H.; Stenhagen, G. J. Chromatogr. 1985, 323,47-66. (35) Tsuda, T.; Nakagawa, G. J. Chromatogr. 1980, 199, 249-258.
.
L I B R A R Y DATA 214
172 187
Figure 5. Corresponding mass spectra of propazine as in Figure 3.
source pressure of about lo4 to torr which is typical for E1 conditions. The chromatogram of RIC in Figure 3 is a very fine one. The theoretical plate number of the second peak is 30000. In our present system, there was no frit a t the column end or inlet of the interface, because supports in a packed microcapillary column had been kept well by the inner glass wall
RECEIVED for review February 25,1985. Accepted March 22, 1985. Part of this paper was presented at the International Symposium on High Performance Liquid Chromatography, Kyoto, Japan, Jan 28-30, 1985. One of the authors was supported by a grant from Deutscher Akademischer Austauschdienst.
